Abrasive tools have long been used in numerous applications, including cutting, drilling, sawing, grinding, lapping and polishing of materials. Because diamond is the hardest abrasive material currently known, it is widely used as a superabrasive on saws, drills, and other devices, which utilize the abrasive to cut, form, or polish other hard materials.
Diamond tools are particularly indispensable for applications where other tools lack the hardness and durability to be commercially practical. For example, in the stone industry, where rocks are cut, drilled, and sawed, diamond tools are about the only tools that are sufficiently hard and durable to make the cutting, etc., economical. If diamond tools were not used, many such industries would be economically infeasible. Likewise, in the precision grinding industry, diamond tools, due to their superior wear resistance, are uniquely capable of developing the tight tolerances required, while simultaneously withstanding wear sufficiently to be practical.
A typical superabrasive tool, such as a diamond saw blade, is manufactured by mixing diamond particles (e.g., 40/50 U.S. mesh saw grit) with a suitable metal support matrix powder (e.g., cobalt powder of 1.5 micrometer in size). The mixture is then compressed in a mold to form the right shape (e.g., a saw segment). This “green” form of the tool is then consolidated by sintering at a temperature between 700–1200° C. to form a single body with a plurality of abrasive particles disposed therein. Finally, the consolidated body is attached (e.g., by traditional brazing or soldering) to a tool body; such as the round blade of a saw, to form the final product.
Despite their prevailing use, diamond tools generally suffer from several significant limitations, which place unnecessary limits on their useful life. For example, the abrasive diamond or cubic boron nitride (CBN) particles are not distributed uniformly in the matrix that holds them in place. As a result, the abrasive particles are not positioned to maximize efficiency for cutting, drilling, grinding, polishing, etc.
The distance between diamond or CBN abrasive particles determines the work load each particle will perform. Improper spacing of the diamond or CBN abrasive particles typically leads to premature failure of the abrasive surface or structure. Thus, if the diamond/CBN abrasive particles are too close to one another, some of the particles are redundant and provide little or no assistance in cutting or grinding. In addition, excess particles add to the expense of production due the high cost of diamond and cubic boron nitride. Moreover, these non-performing diamond or CBN particles can block the passage of debris, thereby reducing the cutting efficiency. Thus, having abrasive particles disposed too close to one another adds to the cost, while decreasing the useful life of the tool.
On the other hand, if abrasive particles are spaced too far apart, the workload (e.g., the impact force exerted by the work piece) for each particle becomes excessive. The sparsely distributed diamond or CBN abrasive particles may be crushed, or even dislodged from the matrix into which they are disposed. The damaged or missing abrasive particles are unable to fully assist in the workload. Thus, the workload is transferred to the surviving abrasive particles. The failure of each abrasive particle causes a chain reaction which soon renders the tool ineffective to cut, drill, grind, etc.
Different applications may require different size of diamond (or cubic boron nitride) abrasive particles. For example, drilling and sawing applications may require a large sized (20 to 60 U.S. mesh) diamond grit to be used in the final tool. The metal substrate of the tool is typically selected from cobalt, nickel, iron, copper, bronze, alloys thereof, and/or mixtures thereof. For grinding applications, a small sized (60/400 U.S. mesh) diamond grit (or cubic boron nitride) is mixed with either metal (typically bronze), ceramic/glass (typically a mixture of oxides of sodium, potassium, silicon, and aluminum) or resin (typically phenolic).
Often the tool may include a matrix support material, such as a metal powder, which holds or supports the diamond particles. However, because diamond or cubic boron nitride is much larger than the matrix powder (300 times in the above example for making saw segments), and it is much lighter than the latter (about ⅓ in density for making saw segments), it is very difficult to mix the two to achieve uniformity. Moreover, even when the mixing is thorough, diamond particles can still segregate from metal powder in the subsequent treatments such as pouring the mixture into a mold, or when the mixture is subjected to vibration. The distribution problem is particularly troublesome for making diamond tools when diamond is mixed in the metal support matrix.
There is yet another limitation associated with the many methods of positioning diamond grits in a tool. Many times a metal bond diamond tool requires different sizes of diamond grits and/or different diamond concentrations to be disposed at different parts of the same diamond tool. For example, saw segments tend to wear faster on the edge or front than the middle. Therefore, higher concentrations and smaller diamond grit are preferred in these locations to prevent uneven wear and thus premature failure of the saw segment. These higher concentration/smaller size segments (i.e. “sandwich” segments) are difficult to fabricate by mixing diamond particles with metal powder. Thus, despite the known advantages of having varied diamond grit sizes and concentration levels, such configurations are seldom used because of the lack of a practical method of making thereof.
Another drawback of many diamond tools is that the abrasive particles, or “grits” are insufficiently attached to the tool substrate, or matrix support material, to maximize useful life of the cutting, drilling, polishing, etc., body. In fact, in most cases, diamond grits are merely mechanically embedded in the matrix support material. As a result, diamond grits are often knocked off or pulled out prematurely. Moreover, the grit may receive inadequate mechanical support from the loosely bonded matrix under work conditions. Hence, the diamond particles may be shattered by the impact of the tool against the workpiece to which the abrasive is applied.
It has been estimated that, in a typical diamond tool, less than about one tenth of the grit is actually consumed in the intended application (i.e. during actual cutting, drilling, polishing, etc). The remainder is wasted by either being leftover when the tool's useful life has expired, or by being pulled-out or broken during use due to poor attachment and inadequate support. Most of these diamond losses could be avoided if the diamond particles can be properly positioned in and firmly attached to the surrounding matrix.
In order to maximize the mechanical hold on the diamond grits, they are generally buried deep in the substrate matrix. As a result, the protrusion of the diamond particles above the tool surface is generally less than desirable. Low grit protrusion limits the cutting height for breaking the material to be cut. As a result, friction increases and limits the cutting speed and life of the cutting tool.
In order to anchor diamond grit firmly in the support matrix, it is highly desirable for the matrix to form carbide around the surface of the diamond. The chemical bond so formed is much stronger than the traditional mechanical attachment. The carbide may be formed by reacting diamond with suitable carbide formers such as a transition metal. Typical carbide forming transition metals are: titanium (Ti), vanadium (V), chromium (Cr), zirconium (Zr), molybdenum (Mo), and tungsten (W).
The formation of carbide requires that the carbide former be deposited around the diamond and that the two subsequently be caused to react to form carbide. Moreover, the non-reacted carbide former must also be consolidated by sintering or other means. All these steps require treatment at high temperatures. However, diamond may be degraded when exposed to a temperature above about 1,000° C. The degradation is due to either the reaction with the matrix material or the development of micro-cracks around metal inclusions inside the crystal. These inclusions are often trapped catalysts used in the formation of synthetic diamond.
Most carbide formers are refractory metals so they may not be consolidated below a temperature of about 1,200° C. Hence, refractory carbide formers are not suitable as the main constituent of the matrix support material.
There are, however, some carbide formers that may have a lower melting temperature, such as manganese (Mn), iron (Fe), silicon (Si), and aluminum (Al). However, these carbide formers may have other undesirable properties that prohibit them from being used as the primary constituent of the matrix support material. For example, both manganese and iron are used as catalysts for synthesizing diamond at high pressure (above 50 Kb). Hence, they can catalyze diamond back to graphite during the sintering of the matrix powder at a lower pressure. The back conversion is the main cause of diamond degradation at high temperature.
Aluminum, on the other hand, has a low melting point (660° C.), thus, making it easy to work with for securing the diamond particles. However, the melting point of aluminum can be approached when diamond grit is cutting aggressively. Hence, aluminum may become too soft to support the diamond grit during the cutting operation. Moreover, aluminum tends to form the carbide Al4C3 at the interface with diamond. This carbide is easily hydrolyzed so it may be disintegrated when exposed to coolant. Hence, aluminum typically is not a suitable carbide former to bond diamond in a matrix.
To avoid the high temperature of sintering, carbide formers, such as tungsten, are often diluted as minor constituents in the matrix that is made of primarily either Co or bronze. During the sintering process, there is a minimal amount, if any, of liquid phase formed. The diffusion of carbide former through a solid medium toward diamond is very slow. As a result, the formation of carbide on the surface of diamond is negligible. Therefore, by adding a carbide former as a minor matrix constituent, the improvement of diamond attachment is marginal at best.
In order to ensure the formation of carbide on the surface of diamond, the carbide former may be coated onto the diamond before mixing with the matrix powder. In this way, the carbide former, although it may be a minor ingredient in the matrix, can be concentrated around diamond to form the desired bonding.
The coating of diamond may be applied chemically or physically. In the former case, the coated metal is formed by a chemical reaction, generally at a relatively high temperature. For example, by mixing diamond with carbide formers such as titanium or chromium, and heating the mixture under a vacuum or in a protective atmosphere, a thin layer of the carbide former may be deposited onto the diamond. Increasing temperature may increase the thickness of the coating. The addition of a suitable gas (e.g. HCl vapor) that assists the transport of the metal may also accelerate the deposition rate. Alternatively, the coating may be performed in a molten salt.
In addition to sintering, infiltration is also a common technique for making diamond tools; in particular for drill bits and other specialty diamond tools that contain large (i.e. greater than U.S. mesh 30/40) diamond grit. Most commonly used infiltrants for these tools are copper based alloys. These infiltrants must flow and penetrate the small pores in the matrix powder. In order to avoid the diamond degradation at high temperature, the melting point of the infiltrant must be low. Hence, the infiltrant often contains a low melting point constituent, such as zinc (Zn). In addition to lowering the melting point of the infiltrant, the low melting point constituent also reduces the viscosity so the infiltrant can flow with ease. However, as most carbide formers tend to increase the melting point of the infiltrant, they are excluded from most infiltrants. As a result, these infiltrants cannot improve the bonding of diamond.
One specific process that has become dependent on the use of diamond tools is chemical mechanical polishing (CMP). This process has become standard in the semiconductor and computer industry for polishing wafers of ceramics, silicon, glass, quartz, etc. In general terms, the work piece to be polished is held against a spinning polishing pad of polyurethane, or other suitable material. The top of the pad holds a slurry of acid and abrasive particles, usually by a mechanism such as fibers, or small pores, which provide a friction force sufficient to prevent the particles from being thrown off of the pad due to the centrifugal force exerted by the pad's spinning motion. Therefore, it is important to keep the top of the pad as flexible as possible, and to keep the fibers as erect as possible, or to assure that there are an abundance of open and pores available to receive new abrasive particles.
A problem with maintaining the top of the pad is caused by an accumulation of polishing debris coming from the work piece, abrasive slurry, and polishing disk. This accumulation causes a “glazing” or hardening of the top of the pad, and significantly decreases the pad's overall polishing performance. Therefore, attempts have been made to revive the top of the pad by “combing” or “cutting” it with various devices. This process has come to be known as “dressing” or “conditioning” the CMP pad. The device most widely used for pad dressing is a disk with a plurality of super hard crystalline particles, such as diamond particles or cBN particles attached thereto.
Dressing disks made by conventional methods share several problems with other superabrasive tools, made by conventional methods. However, such issues may have a much greater impact on the CMP process. For example, poor superabrasive grit retention may lead to scratching and ruining of the work piece. Uneven work loading of the superabrasive grits resulting from clustered or unevenly spaced particle groups may cause overdressing of certain pad areas and under dressing of others, which results in unsuitable work piece polishing. Moreover, when the superabrasive particles of dressing disks do not extend to a uniform height above the substrate surface of the disk uneven dressing of the CMP pad is further propagated, because many particles from the dresser may not touch the pad.
In addition to the above-recited issues with particle retention and distribution, the CMP pad dressing process itself creates additional issues that make uncontrolled superabrasive particle placement unacceptable. For example, the downward pressing force of a dressing disk on a CMP may depress the pad upon contact with the leading edge of the dresser, and prevent the remaining superabrasive particles on the pad dresser from sufficiently contacting the pad to achieve even dressing.
Warping of the pad dresser working surface during the brazing process also often causes abrasive particles to dislodge. During the brazing process the pad dresser must be exposed to very high temperatures. Exposure to this extreme heat can cause the working surface of the pad dresser to warp, thus compromising the smoothness and planarity of the pad dresser's working surface. As a result, the braze portion of the working surface will be rough, having high and low spots. Such spots are undesirable, as they may cause the braze to begin flaking off, and making micro-scratches on the polished surface of the work piece.
As a result, suitable methods of maximizing the efficiency, useful life, and other performance characteristics of diamond tools are continually being sought.